FIGS. 8a and 8b are a side cross-sectional view and a top plan view, respectively of a liquid discharge mechanism according to a conventional technique as described in Japanese Unexamined Patent Publication No. 2000-146089. The liquid discharge mechanism shown in these drawings has an inner lid 111 arranged on a mouth of a liquid tank not shown. Two female screw holes 112 and 113 are formed on the inner lid 111. A plug 114 for supplying pressurized gas and a plug 115 for discharging liquid are screwed in the female screw holes 112 and 113, respectively.
As can be seen from FIG. 8b, plural flat surfaces 122 and 134, respectively, for example, four flat surfaces 122 and 134, respectively, are formed at each equal angle, around circumferential surfaces of the plugs 114 and 115, respectively. The plugs 114 and 115 are aligned so that each one flat surface 122 of the plug 114 faces in parallel with each one flat surface 134 of the plug 115. A rotation prevention member 124 that prevents a rotation of the plugs 114 and 115 is laid out between the mutually opposite flat surfaces 122 and 134, and is fastened to the top surface of the inner lid 111 with a screw 160. With this arrangement, a flat surface 123 of the rotation prevention member 124 is brought into contact with the flat surfaces 122 and 134 of the plugs 114 and 115, respectively, thereby preventing a rotation of the plugs 114 and 115. Usually, the shown liquid discharge mechanism is held on the mouth of the liquid tank with a circular cap (not shown). When gas is supplied from the plug for supplying pressurized-gas into the liquid tank, the liquid within the liquid tank is discharged through the plug for discharging liquid.
However, in the liquid discharge mechanism of the conventional technique as disclosed in Japanese Unexamined Patent Publication No. 2000-146089, each of the flat surfaces 122 and 134 with which the rotation prevention member 124 is in contact is smaller than a quarter of the entire circumferential surface of the corresponding plugs 114 and 115, respectively. Therefore, the force of the rotation prevention member 124 for holding the plugs 114 and 115 is relatively small. Because the rotation prevention member 124 is merely fastened with the screw 160, when the screw 160 is loosened, the plugs 114 and 115 are rotated and loosened, and are likely to be separated from the inner lid 111.
Although not shown in FIG. 8a, front ends of the plugs 114 and 115 are connected to a socket for supplying pressure-gas and a socket for discharging liquid, respectively. At the time of disconnecting these sockets, an operator may pull these sockets while swinging the sockets back and forth, without directly pulling up the sockets. In this case, the screw 160 of the rotation prevention member 124 is loosened. Thus, the plugs 114 and 115 are also rotated and loosened, and are likely to be separated from the inner lid 111.
The present invention has been made in the light of the above problem, and has an object of providing a loosening prevention device for a plug, which makes it possible to firmly hold the plug for supplying pressurized-gas and for discharging liquid.